AFAIK light text on dark background might improve contrast between different colors, which is useful when coding; however, for better overall contrast and readability, dark on white is preferable.

- Various studies suggest that white on black is easier to read and comprehend, which is important in presentations, especially for people who stand far from screen.

See: http://stackoverflow.com/questions/498698/white-light-vs-bla..., http://www.codinghorror.com/blog/2005/07/code-colorizing-and...

- In printed media, the general recommendation is to use larger font for text displayed on dark or complex backgrounds, as it's considered harder to read. (This is from my experience and what they taught us at university, so no references here.)

Also, if there's need to include inverse text which should look to reader the same size as regular one printed elsewhere on white, the slightly larger / ‘bolder’ font is used. The same size font would actually be perceived as smaller.

(Edit: formatting.)